New concepts on formation of Brunn's nests and cysts in urinary tract mucosa.

To the present time, Brunn's nests have been conceived as being the result of pinching off of epithelial nests from urothelial buds. Mucosal cysts were considered to originate from Brunn's nests. In the present study, it was observed that under the influence of a strong stimulus of repair, the urothelial cells in the proximity of the focus of repair undergo numerous mitoses. The underlying connective tissue becomes loose, isolated cells of the lining urothelium "drop off" into the lamina propria in which they migrate as in a culture medium. There they multiply and form nests of cells at a distance from the lining urothelium. These migrating cells frequently have an early tendency to become arranged into cysts. Thus, during this process of repair urothelial cells behave temporarily as malignant cells.